1. Field of the Invention
The present invention relates to a surface acoustic wave element. More particularly, the present invention relates to a surface acoustic wave element including electrodes that have high electric power durability, and are suitable for use in antenna duplexers, and other electronic devices.
2. Description of the Related Art
Surface acoustic wave elements are usually prepared by forming a comb-like interdigital transducer electrodes (referred to as xe2x80x9cIDT electrodexe2x80x9d, hereinafter) including an Al film on the surface of a substrate having piezoelectric characteristics. Such surface acoustic wave elements are widely used for resonators, inter-stage filters, duplexers, etc.
In recent years, development of miniaturized and much lighter mobile communication terminal devices such as portable telephones has advanced rapidly. Accordingly, components for use in such mobile communication terminal devices have been required to be miniaturized. In relation to this, demands for miniaturized surface acoustic wave elements for use in resonators, inter-stage filters, duplexers, etc. have been increasing rapidly, since they can contribute to miniaturization of RF devices (high frequency devices). More particularly, high electric power durability is required for antenna duplexers located at the front end of RF devices.
Furthermore, as the mobile communication devices are being used in higher frequency ranges, it is necessary to make surface acoustic wave devices (SAW devices) using surface acoustic wave elements operate in higher frequency ranges of several hundred MHz to several GHz, and with high output.
In order to make the surface acoustic wave devices operate in higher frequency ranges, it is necessary to realize finer pattern widths for IDT electrodes. For example, filters with a central frequency in a 2 GHz range require finer electrode lines with a line width that is reduced down to about 0.5 Πm. In order to make the surface acoustic wave devices operate with a high output, it is necessary to form electrodes that are not damaged due to generation of hillocks, voids, and other problems, even when signals at higher voltage levels are applied to the surface acoustic wave elements.
However, when signals at higher voltage levels are applied to surface acoustic wave elements having such a fine line width, the surface acoustic waves exert a high stress on their IDT electrodes that are made of Al film, for example. When the stress exceeds the stress limit of the IDT electrodes (Al films), stress migration will occur, causing Al atoms as an electrode material to migrate through the crystal grain boundary, leading to generation of hillocks, voids, and other problems, and resulting in destruction of the IDT electrodes, increase of electric short circuit and insertion loss, and deterioration in properties including a lowered Q factor of resonators.
In order to solve these problems, Japanese Unexamined Patent Application Publication No. 7-122961 proposes a surface acoustic wave element having electrodes formed by laminating Al alloy films with at least Cu added thereto and Cu films alternately on a piezoelectric substrate. Japanese Unexamined Patent Application Publication No. 9-69748 proposes a SAW device, wherein Al films and films composed of an electroconductive material having an elastic modulus larger than that of the Al films are laminated alternately, and the number of the laminated films are not less than two, respectively. In both cases, it is believed that the films improve the electric power durability.
However, in recent years, the line width of electrodes for surface acoustic wave elements is becoming finer and finer, while even higher output is required. Thus, there are cases in which the above-described conventional multilayer electrodes cannot fully satisfy such requirements.
In order to overcome the problems described above and meet the ever-increasing demands of the electronics industry, preferred embodiments of the present invention provide a surface acoustic wave element having electrodes (multilayer electrodes) with greatly improved electric power durability, compared to the conventional surface acoustic wave elements.
According to a preferred embodiment of the present invention, a surface acoustic wave element includes a piezoelectric substrate and an electrode disposed on the piezoelectric substrate, wherein the electrode is a multilayer electrode having three or more layers, the electrode including (a) a base electrode film disposed on the piezoelectric substrate; (b) an Al-type electrode film including Al as a main component that is disposed on the base electrode film; and (c) an electrode film having a diffusive material as a main component that is disposed on the Al-type electrode film, the diffusive material being able to easily diffuse into the grain boundary of the Al-type electrode film, and wherein the base electrode film includes a material that is capable of improving the orientation characteristics of the multilayer electrode.
By making the electrode having three or more layers including (a) a base electrode film disposed on the piezoelectric substrate, (b) an Al-type electrode film including Al as a main component that is disposed on the base electrode film, and (c) an electrode film having a diffusive material as a main component that is disposed on the Al-type electrode film, the diffusive material being able to easily diffuse into the grain boundary of the Al-type electrode film, it is possible to orientate the electrode (multilayer electrode) along one axis, or even along three axes to achieve higher degree of orientation, resulting in greatly improved electric power durability. It is possible to further improve the electric power durability by virtue of the diffusion of the diffusive material in the electrode film disposed on the Al-type electrode film into the grain boundary of the Al-type electrode films.
Furthermore, a surface acoustic wave element according to another preferred embodiment of the present invention is arranged such that the above-described base electrode film includes, as a main component, Ti that is capable of improving the orientation characteristics of the above-described multilayer electrode, and the above-described Al-type electrode is orientated along three axes.
When the base electrode film includes, as a main component, Ti that is capable of improving the orientation characteristics of the multilayer electrode, and the Al-type electrode is orientated along three axes, it is possible to further improve the electric power durability. This is another advantage of preferred embodiments of the present invention.
Furthermore, a surface acoustic wave element according to still another preferred embodiment of the present invention is characterized in that the electrode film having a diffusive material includes, as a main component, at least one metal selected from the group consisting of Cu, Ag, Au, Ni, and Mg that can easily diffuse into the grain boundary of the above-described Al-type electrode film.
By forming, as an electrode film disposed on the Al-type electrode film, an electrode film including, as a main component, at least one metal selected from the group consisting of Cu, Ag, Au, Ni, and Mg that can easily diffuse into the grain boundary of the Al-type electrode film, it is possible to achieve further improvement of the electric power durability.
Furthermore, a surface acoustic wave element according to still another preferred embodiment of the present invention is characterized in that each film layer constituting the above-described multilayer electrode has a film thickness of not more than approximately 50 nm.
By making the film thickness of each film layer constituting the multilayer electrode not more than approximately 50 nm, it is possible to prevent grain growth in the direction of the plane of the crystal grains and to prevent the diffusive material from diffusing into the grain boundary non-uniformly, resulting in further improvement in the electric power durability of the multilayer electrode.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.